One type of system for producing hydrocarbons from undersea reservoirs of limited capacity, includes a floating structure such as a vessel anchored by catenary chains to the seafloor, or spread moored, or otherwise moored in a manner that allows limited vessel drift. Hydrocarbons from a seafloor well tapped into the reservoir, flow through conduits of a conduit structure, that extend up to the vessel to fill tanks in the vessel. Fluids such as injected gas may be pumped downward through a conduit back into the reservoir. Additional connections such as electrical and hydraulic connections may extend from the vessel to apparatus at the seafloor. The conduit structures must continue fluid connections between the vessel and seafloor well(s) despite drifting of the vessel within a limited drift zone. The conduits should not hit the mooring chains or the seafloor, since this can cause wear of a conduit.
One prior art conduit structure includes a first flexible hose that extends almost vertically up from the seafloor to an underwater buoy, and a second flexible hose that extends in a double catenary curve from the buoy to the vessel. In moderate to deep water (e.g. about 100 meters or more) the buoy lies high above the seafloor and the double catenary second hose provides a connection during vessel drift. However, a considerable length of hose is required, and flexible hose is expensive and not as reliable as a fixed pipe. In shallow water, any underwater buoy must lie close to the seafloor, resulting in appreciable cost for the buoy, for a heavy seafloor weight to moor the buoy, and for hose connections of a short first hose. In addition, a buoy at shallow depths moves sideward in heavy waves, in directions that may be counter to vessel movement, and the moveable parts limit the reliability of a buoy-based conduit system in shallow water. A fluid transfer system for transferring fluids between a seafloor structure and a floating structure in shallow water, which was of minimal cost while providing reliable connections during vessel drift, without a conduit beating against an anchor chain or the seafloor, would be of value.